1. Field of the Invention
The invention is in the field of chemical process control and, more specifically, in the field of polyolefin polymerization process control.
2. Description of the Prior Art
Manufacture of numerous types of thermoplastic olefin polymers now is well known and routinely commercially practiced based on Ziegler-Natta catalyst systems. Useful commercial manufacturing processes for olefin polymers using Ziegler-Natta catalysts have evolved from complex slurry processes using an inert hydrocarbon diluent, to efficient bulk processes using liquid propylene diluent, to even more efficient gas-phase processes in which solid polymer is formed directly from polymerizing gaseous olefin monomer.
Typically-used gas-phase processes include horizontally and vertically stirred sub-fluidized bed reactor systems, fluidized bed systems, as well as multi-zone circulating reactor systems. Thermoplastic olefin polymers made in these processes include polymers of ethylene and C3-C10+ alpha-olefin monomers and include copolymers of two or more of such monomers, such as statistical (random) copolymers or multi-phasic (rubber-modified or impact) copolymers.
Polymers of propylene, which contain crystalline polypropylene segments, are advantageously produced in the gas phase. Such propylene polymers include polypropylene homopolymer in which essentially all of the monomer units are propylene and copolymers of propylene with up to fifty mole percent (50 mole %) of one or more of ethylene or C4+ olefin monomer. Usually, propylene/ethylene copolymers contain up to about 30 wt. %, typically up to about 20 wt. %, of ethylene monomer units. Depending on the desired use, such copolymers may have a random or statistical distribution of ethylene monomer units or may be composed of an intimate mixture of homopolymer and random copolymer chains, typically referred to as rubber-modified or impact copolymers. In such rubber-modified or impact copolymers, typically a high ethylene content random copolymer functions as an elastomeric or rubber component to alter the impact properties of the combined polymer material.
The molecular weight of an olefin polymer, especially propylene polymers, typically is regulated by the use of hydrogen in the polymerization gas mixture. Generally speaking, a higher concentration of hydrogen will result in a lower molecular weight. The molecular weight distribution of the polymer composition, sometimes referred to as polydispersity, may affect polymer properties.
In horizontal stirred reactors, the average value of the distribution can be controlled by adjusting the inlet hydrogen flow rate to maintain a constant hydrogen to propylene ratio in the off-gas of the reactor. There is a direct link between the average chain length of the final polymer product and the gas phase hydrogen to propylene ratio. Known prior art processes cannot control molecular weight distribution broadness, but it is known that it varies slightly whatever the process conditions of operations.
The purpose of the instant invention is therefore to deal with the broadening of the molecular weight distribution of the polymer made in a horizontal stirred reactor. By applying a hydrogen gradient along the horizontal stirred reactor, the molecular weight distribution can be broadened and controlled in a large range of polydispersity indexes.
Polymer compositions containing polymer components with different physical properties have been found to have desirable properties. Thus, total polymer compositions containing different amounts of individual polymers in a multimodal distribution may result in a polymer with properties which are distinct from any of the polymer components. A conventional method of producing such multimodal polymers is to blend individual polymers by physical means, such as a blender or blending extruder. A more efficient method of obtaining a multimodal product composition is to produce the product directly in polymerization reactors. In such in situ production, a more intimate mixture may be produced, which results in more advantageous properties than are able to be produced by physical blending.
Producing a multimodal product typically requires a process in which polymerization occurs with different conditions at different times or places in the process. Although a single reactor may be used in a batch process to simulate a multi-reactor continuous process, typically batch processes are not practical commercially. A multi-reactor system may be used, which uses two or more reactor vessels.
Gas-phase or vapor-phase olefin polymerization processes are disclosed generally in “Polypropylene Handbook” pp. 293-298, Hamer Publications, NY (1996), and more fully described in “Simplified Gas-Phase Polypropylene Process Technology” presented in Petrochemical Review, March, 1993. The disclosures of these publications are hereby incorporated herein by reference.
As described in U.S. Patent Application No. 2012/00951712, the disclosure of which is hereby incorporated by reference, it is possible to use a hydrogen gradient in a horizontal stirred bed reactor to produce polyolefins having a broad molecular weight distribution.
There is a need for an olefin polymerization process in which product composition may be controlled, especially among different polymerization zones. Also, there is a need for a polymerization process which is able to broaden and control the molecular weight distribution of the polymer made in a stirred horizontal reactor.
The instant invention comprises a method of controlling a polymerization process that creates a hydrogen gradient within the reactor. Polymers of very different molecular weight are then produced leading to a broadened molecular weight distribution. The homopolymers made in a single reactor under these “hydrogen gradient” conditions have shown better processability and higher melt strength than previously known processes.
Thanks to the broadening of the molecular weight distribution, several final properties of the polymer are enhanced without any detrimental effects on other properties. In addition, the process of the invention makes it possible to make new products (or enhanced products) by operating the process under new conditions.